1. Field of the Invention
The present invention relates to a camera having an automatic focusing system, to which a photographing lens can be attached.
2. Description of the Related Art
A single lens reflex camera having an automatic focusing system (referred to as an AF single lens reflex camera) in which an amount of defocus for an object to be photographed is detected, so that a focusing lens group is moved to eliminate the defocus to thereby focus on the object, is known. In the known AF single lens reflex camera, the camera body is provided therein with a focusing system (object distance measuring device, defocus detecting device) and a lens drive motor for driving the focusing lens group of the photographing lens, etc., so that when the photographing lens is attached to the camera body, the focusing lens group can be moved by the lens drive motor in accordance with the amount of defocus detected by the defocus detecting device.
In the automatic focusing operation in a known AF single lens reflex camera, if the object is moving, that is, if the distance of the object from the camera changes, there is a possibility that a no in-focus state is obtained by the movement of the focusing lens group by the lens drive motor in accordance with the amount of defocus detected by the defocus detector. This is because the object moves close to or away from the camera after the detection of the focus, but before the completion of the movement of the focusing lens group. Moreover, there is a certain period of time (release time lag) between the depression of the release button and the commencement of the movement of the shutter. Consequently, upon photographing a moving object, even if an in-focus state has been established when the release button is depressed, the object which moves close to or away from the camera in the release time lag could be out-of-focus when the shutter is opened (at the beginning of the exposure).
In recent years, an AF single lens reflex camera having an action mode (focus prediction mode), called a "moving-object prediction mode" or an "object-tracing mode", has been proposed in which the focus can be adjusted in accordance with an algorithm on the action mode, so that the in-focus state can be established upon viewing the moving object or upon the operation of the shutter. In this AF single lens reflex camera, information on the focal length and the displacement of the focusing lens group necessary to move the image surface, on which an object image is formed, by the photographing lens is formed, or the angular displacement of the drive shaft of the lens drive motor, etc., is transmitted from the photographing lens to the camera body.
In general, in an AF single lens reflex camera, the detection of the defocus amount and the movement of the focusing lens group are carried out so as to establish an in-focus state. The defocus detection and the movement of the focusing lens group are repeated until the amount of defocus is zero. If a defocus amount of zero cannot be obtained even after a predetermined number of defocus detection operations and focusing-lens-group moving operations have been carried out, the object is then deemed as a moving object. Consequently, the control enters the moving-object mode in which the moving velocity of the object image surface is calculated based on the previous defocus amount and the current defocus amount detected when the focusing lens group was moved by a displacement corresponding to the previous defocus amount. Consequently, the lens drive motor is driven in accordance with the defocus amount which has been obtained, taking into account the moving velocity of the image surface, the space of time between the movement of the focusing lens group and the detection of the defocus amount.
If the release button is depressed when in the moving-object mode, the defocus amount caused by the movement of the moving object in the release time lag from the mirror-up operation to the commencement of the movement of the shutter blinds is predicted, so that the focusing lens group is moved in accordance with the predicted defocus amount to establish a nearly in-focus state when the shutter blinds are opened.
However, in some photographing lenses in which the change in the movement of the image surface relative to the angular displacement of the drive shaft of the lens drive motor (drive amount of the drive motor) when the focal length or the object distance varies is large, the above-mentioned solution is not useful. In this case, the drive amount (unit drive value) of the drive motor necessary to move the image surface by a unit displacement must be set to a minimum value to prevent over-movement. Consequently, there is a possibility that an in-focus state is not obtained by the number of the focus detection operations and the lens movement operations corresponding to the number of moving-object mode entries that are executed, so that even if the object stops moving, the object is judged to be a moving object. Thus, the control enters the moving-object mode. As a result, it is impossible to stop the focusing lens group at the focal position, and hence the focusing lens group moves beyond the focal position. Consequently, hunting occurs.
It is an object of the present invention to provide a single lens reflex camera having an automatic focusing system, to which various types of photographing lenses can be detachably mounted, wherein the entry into the moving-object mode is prohibited depending on the characteristics of the photographing lens to be mounted.